A last-ditch effort to prevent yellow fever spreading through Kinshasa in the Democratic Republic of the Congo and potentially developing into a global epidemic is to be launched using vaccines containing a fifth of the normal dose because the global stockpile is so low.

Fighting yellow fever in the Democratic Republic of the Congo – in pictures

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Yellow fever is frequently lethal, killing half of those who develop severe symptoms. It is transmitted by the bite of the Aedes aegypti mosquito, which is also responsible for the spread of Zika virus. There is a vaccine which protects people for life, but few adults had been immunised in Angola when yellow fever broke out there in December last year, and in the DRC, to where it has spread.

If it takes hold in Kinshasa, a densely packed city of more than 10 million people, it is feared that infected mosquitoes could travel beyond the central African region, which has been experiencing so severe an outbreak that vaccine stocks are almost exhausted.

In Angola, there have been nearly 4,000 suspected cases of yellow fever of which 879 have been confirmed while DRC had 68 confirmed cases and more than 2,200 suspected cases, with about 400 reported deaths in the two countries, mostly in Angola. Almost 19m doses of vaccine have been administered since January, but there are only 5m left in the emergency stockpile. The vaccine takes a year to make, so even with the handful of manufacturers working flat out, stocks cannot be replenished quickly.

“It has got us incredibly worried,” said Ruairidh Villar of Save the Children, which is helping with the vaccination effort. “We’ve just scrambled an emergency team to DRC to support a last-ditch vaccination campaign before the outbreak reaches Kinshasa. Our fear is it is likely to go global if we can’t stop it soon and it hits the city.”

He added: “It is not as fatal as Ebola but it is pretty awful. The symptoms include fever, headache, jaundice, muscle pain, nausea, vomiting and fatigue. Some people recover within a few days but a minority become severely ill, suffering organ failure, bleeding from the eyes and death.

Members of a Médecins Sans Frontières team fumigate the Yolo Sud neighbourhood of Kinshasa. Photograph: Jerome Delay/AP

Some say the spread of yellow fever shows the world has not learned the lessons of Ebola.

“Yellow fever is something that we know and it is something for which we have the tools to prevent the spread, which is the vaccine,” said Dr Joanne Liu, international president of Médecins Sans Frontières, which is involved in the yellow fever response. “The first cases of yellow fever were in December 2015 and confirmed in January. The first vaccination started a few weeks afterwards, but it could have been a bit more prompt.

“What does it tell us? Even if we have the best surveillance systems and tools, we still need the political will to act at local level, regional level and international level. As with Ebola, when that is not enacted, we always, always have a delay in the response. There were some critical weeks which basically allowed fertile ground for spread.

“This makes me deeply saddened and also a little bit in despair. We need to act – a quick and prompt response.”

There have been reports of 1m doses of vaccine disappearing after being delivered to Angola, but the World Health Organisation denies any doses went missing. The government appears to have used the vaccine elsewhere in Angola.

“Following a review of the first round of vaccination campaigns in Angola, it was found that around 1m doses from the emergency stocks had been used to vaccinate people in other medical facilities that were not part of the initial approved emergency vaccination plans,” a WHO spokesman said.

A mass vaccination campaign in western DRC in May immunised more than 200,000 people, said Heather Kerr, Save’s country director there. But it did not stop the spread. “Now we’re planning a second response with the emergency health unit. We work together with them to support the government. The plan is to vaccinate in all the provinces bordering Angola because the yellow fever originally arrived from Angola,” Kerr said by Skype from Kinshasa.

Time is running out to prevent the spread of yellow fever because the rains will begin in September. Photograph: Jerome Delay/AP

“Kinshasa is such a huge city. We don’t really know how big it is. We think there are about 10 million people here. Some of the suburbs are very, very crowded. We are close to the Angolan border as well. There have already been cases in Kinshasa – not many, but a few – because of the density of the population and the ability of the mosquitoes to travel far.

“Fortunately for us, we’re in the dry season but there are still people being bitten by mosquitoes and not everybody is sleeping under a mosquito net.”

WHO said that, until now, outbreaks in central Africa had been in rural areas, not crowded urban environments, where mosquitoes thrive in standing water. Time is running out to check the spread, however, because the rains will begin in September.

The emergency vaccination campaign beginning on Wednesday was one of the largest ever attempted, WHO said.

“Working with ministries of health in the two countries, WHO is coordinating 56 global partners to vaccinate more than 14 million people against yellow fever in more than 8,000 locations,” it said in a statement. “The yellow fever outbreak has found its way to dense, urban areas and hard-to-reach border regions, making planning for the vaccination campaign especially complex.”

The decision to give people one-fifth of the normal dose of vaccine – known as fractional dosing – was taken by WHO’s strategic advisory group of experts on immunisation (Sage). There are some small studies that show it gives protection for at least a year and possibly longer.

″Protecting as many people as possible is at the heart of this strategy. With a limited supply we need to use these vaccines very carefully,″ said William Perea, coordinator for the control of epidemic diseases unit at WHO.